hereditary thrombocytopenias: a growing list of...

| IT ALL STARTS HERE: DISORDERS OF PRIMARY HEMOSTASIS |

Hereditary thrombocytopenias: a growing list of disorders

Patrizia Noris and Alessandro Pecci

Department of Internal Medicine, IRCCS Policlinico San Matteo Foundation and University of Pavia, Pavia, Italy

The introduction of high throughput sequencing (HTS) techniques greatly improved the knowledge of inheritedthrombocytopenias (ITs) over the last few years. A total of 33 different forms caused by molecular defects affecting atleast 32 genes have been identified; along with the discovery of new disease-causing genes, pathogenetic mechanismsof thrombocytopenia have been better elucidated. Although the clinical picture of ITs is heterogeneous, bleeding hasbeen long considered the major clinical problem for patients with IT. Conversely, the current scenario indicates thatpatients with some of the most common ITs are at risk of developing additional disorders more dangerous thanthrombocytopenia itself during life. In particular, MYH9 mutations result in congenital macrothrombocytopenia andpredispose to kidney failure, hearing loss, and cataracts, MPL and MECOM mutations cause congenital thrombocy-topenia evolving into bone marrow failure, whereas thrombocytopenias caused by RUNX1, ANKRD26, and ETV6 mu-tations are characterized by predisposition to hematological malignancies. Making a definite diagnosis of these forms iscrucial to provide patients with themost appropriate treatment, follow-up, and counseling. In this review, the ITs known todate are discussed, with specific attention focused on clinical presentations and diagnostic criteria for ITs predisposingto additional illnesses. The currently available therapeutic options for the different forms of IT are illustrated.

Learning Objectives

• Become familiar with the several inherited thrombocytopeniasidentified so far and recognize forms predisposing to otherdiseases (“predisposing forms”)

• Understand the need to investigate the molecular basis ofpredisposing forms to arrange the most appropriate treatmentand follow-up for each patient

• Be updated on the newest treatments available for patients withthe different forms of inherited thrombocytopenia

IntroductionThe history of inherited thrombocytopenias (ITs) began in 1948 withthe description of a patient suffering from a congenital bleedingdisorder, which was called Bernard-Soulier syndrome (BSS). Almost70 years later, advances in clinical and biomedical research led tothe definition of a total of 33 different forms of IT caused by mo-lecular defects affecting at least 32 genes. Some decades ago, a firstmajor boost to the improvement of knowledge on ITs came from thewidespread diffusion of automated cell counters; they greatly in-creased the identification of patients with thrombocytopenia, even-tually leading to the recognition that the prevalence of ITs is muchhigher than previously thought, affecting at least 2.7:100 000 in-dividuals, as estimated in the Italian population.1 The developmentand diffusion of genetic technologies definitively led to the ex-plosion of knowledge on ITs. Sanger sequencing and linkageanalysis have revealed the molecular bases of some ITs since 1990.Currently, Sanger sequencing is considered a low throughput andtime-consuming approach that is mainly used in patients for whoma diagnostic hypothesis has been formulated based on clinical and

laboratory investigation of their phenotypes. A number of high-throughput sequencing (HTS) technologies, formerly next-generationsequencing (NGS), have been recently developed and widely appliedto human inherited diseases. NGS-targeted platforms allow theanalysis of a predefined group of genes at once, thus making mo-lecular diagnosis of inherited disorders quicker and less expensive. Atargeted sequencing platform, covering 63 genes linked to bleedingand thrombotic disorders, showed 100% sensitivity in detectingcausal variants previously identified by Sanger sequencing and al-lowed detection of the disease-causing gene in 90% of the patientswho had not been previously investigated at the molecular level.2

Amajor improvement in knowledge of ITs came from the introductionof whole exome sequencing (WES) and whole genome sequencing(WGS). They have been adopted by several national and internationalconsortia focused on the identification of the genes responsible for ITin patients who remained without a molecular diagnosis after theanalysis of 1 or more candidate genes.PRKACG,GFI1b, STIM1, FYB,SLFN14, ETV6, DIAPH1, and SRC are examples of disease-causinggenes detected by these HTS approaches.3-11

The findings obtained by both WES and WGS not only improved thedefinition of the phenotypes of several ITs but also inspired a numberof functional studies on the role of molecules coded by the causativegenes, whose function in platelet production was often previouslyunknown. This approach is expected to improve our comprehensionof the physiologic mechanisms of megakaryopoiesis and plateletbiogenesis.

Figure 1 illustrates the different approaches used over the decades fordiscovering themolecular defects responsible for the known forms of IT.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Off-label drug use: Eltrombopag for increasing the platelet count in patients with inherited thrombocytopenias.

Hematology 2017 385

ClassificationThe classifications of ITs that proved most effective for diagnosticpurposes were those based on platelet size, which is greatly variablein the different forms,12 and the presence of further defects in ad-dition to thrombocytopenia.13 Although useful, the effectivenessof the classification by inheritance is affected by the high frequencyof sporadic cases due to de novo mutations in some disorders14 andthe incomplete penetrance of the same mutations, which often makeit difficult to recognize the inheritance pattern.15-17

On the basis of our more recent view on ITs, we propose a classi-fication (Table 1),3-11,14-37 in which ITs are divided into 3 groups:forms characterized by only the platelet defect; disorders in whichthe platelet phenotype associates with additional congenital defects(syndromic forms); and forms characterized by increased risk ofacquiring additional diseases during life. Thus, this classification isuseful for both diagnostic and prognostic purposes.

PathogenesisIn most ITs, the low platelet count derives from defects in 1 or morestep(s) of the complex process of platelet biogenesis,38 whereas ina few disorders, thrombocytopenia results from reduced plateletlifespan. Table 24,5,7-11,17,19,20,22,24,26,27,29,30,32,35,36,39-45 summa-rizes the main pathogenetic mechanisms of ITs.

The key pathogenetic mechanism of some ITs is defective differen-tiation of hematopoietic stem cells (HSCs) into megakaryocytes(MKs), resulting in the absence or severe reduction in the number ofbonemarrowMKs. The clinical picture of congenital amegakaryocyticthrombocytopenia (CAMT) reflects the nonredundant roles of theMPL receptor inMK differentiation since birth and in the maintenanceof the stem cell compartment in postnatal hematopoiesis. Modeling ofhematopoiesis of CAMT by using patient-derived induced pluripotentstem cells showed a failure in the transition of multipotent towardMK progenitors as well as defective self-replication and survival ofmultipotent stem cells; these phenotypes were rescued by transductionof a functional MPL.39

In 11 forms of IT, the main pathogenetic mechanism is altered MKmaturation and therefore the production of a normal or increased

number of immature, dysmorphic, and dysfunctional MKs. Of note,6 of these disorders are caused by mutations of transcription factorswith a key role in megakaryopoiesis, ie, RUNX1, FLI1, GATA1,GFI1b, and ETV6 (Table 2). Each of these transcription factorsregulates, as activator or repressor, the expression of numerous genes;thus, the ITs caused by their defects are characterized by the concurrentalterations of multiple mechanisms of MK and platelet development.For instance, RUNX1 directly transactivates genes encoding for othertranscription factors involved inMKmaturation (NF-E2), componentsof the MK cytoskeleton (MYH9, MYL9, MYH10), proteins implicatedin a and dense granule development (PF4, PLDN), and members ofMK/platelet signaling pathways (ANKRD26,MPL,PRKCQ, ALOX12,PCTP).42 Some studies identified the links between the MK abnor-malities observed in patients with familiar platelet disorder (FPD) withpredisposition to acute myeloid leukemia (AML) and altered ex-pression of specific RUNX1 transcriptional targets. Defective MKpolyploidization is explained by dysregulation of MYH10 and im-paired proplatelet formation by reduced MYH9 and MYL9 expres-sion,46 whereas downregulation of PLDN contributes to defectivedense granule development.47 The erythrocyte abnormalities typical ofthe disorders caused by GATA1 and GFI1b mutations indicate theessential role of these transcription factors in also controlling red cellproduction. Moreover, the predisposition to hematological neoplasmsof patients with the ITs caused by RUNX1 and ETV6 mutationshighlights how these pathogenetic variants also disrupt the homeo-stasis of myeloid and multipotent progenitors, respectively.

In a third group of ITs, the differentiation and maturation of MKs aresubstantially preserved, and thrombocytopenia derives from alter-ations of the extension and release of proplatelets from mature MKsand/or the conversion of proplatelets to platelets within the blood-stream. Most of these disorders are macrothrombocytopenias causedby mutations of genes encoding for components of the actomyosincytoskeleton or microtubule system, such as MYH9, ACTN1, FLNA,TPM4, TRPM7, or TUBB1. This scenario reflects the key role of themechanical forces generated by the cytoskeleton and microtubules indriving the processes of remodeling, protrusion, and fission of the MKcytoplasm, leading to the release of platelets.48 Defective proplateletformation and platelet release are the key pathogenetic mechanismsalso of the macrothrombocytopenias that are caused by mutations ofgenes for the major MK membrane glycoprotein (GP) complexes

Figure 1. Genetic approaches used for discovering the molecular defects responsible for the known forms of inherited thrombocytopenias. For theabbreviations, see Table 1. mTHPO mutation, inherited thrombocytopenia from a monoallelic THPO mutation.

386 American Society of Hematology

Table1.

Mainfeatures

ofinhe

rited

thrombo

cytope

nias

clas

sified

acco

rdingto

theirclinical

picture

Disea

se(abb

reviation,

OMIM

entry)

Inhe

ritan

ceGen

e(lo

cus)

Thrombo

cytope

nia*/p

latelet

size

Bleed

ing†

Add

ition

alfeatures

tothrombo

cytope

nia

Form

swith

only

thrombo

cytope

nia

Berna

rd-Sou

liersynd

rome(bBSS,mBSS,

2312

00/153

670)

Biallelic

18

AR

GP1B

A(17p

13)GPIBB

(22q

11)

11,1

11/giant,large

SIm

paire

dplatelet

func

tion.

Mon

oallelic

15

AD

GP9(3q2

1)1/la

rge

A/M

iGrayplatelet

synd

rome(G

PS,13

9090

)3AR

NBEAL2

(3p2

1)11,1

11/la

rge

Mi/S

Impa

iredplateletfunc

tion.Severelyredu

cedco

nten

tof

agran

ules.P

lateletc

ount

decrea

sesover

time.

Develop

men

tofp

rogressive

bone

marrowfibros

isan

dsp

leno

meg

aly.

Elevatedserum

vitamin

B12

levels.

ACTN

1-relatedthrombo

cytope

nia(ACTN

1-RT,

6151

93)19

AD

ACTN

1(14q

24)

1/la

rge

A/M

i—

Platelet-typevonWilleb

rand

disease(PTvWD,

1778

20)20

AD

GP1B

A(17p

13)

1,1

11/no

rmal,slightly

increa

sed

A/M

iPlateletco

untis

norm

alin

mos

tpa

tientsbu

tmay

decrea

segrea

tlyun

derstressfulc

onditions

(pregn

ancy,surgery,

andinfection).

ITGA2B

/ITGB3-relatedthrombo

cytope

nia

(ITGA2B

/ITGB3-RT(187

800)

21

AD

ITGA2B

(17q

21)

1,1

1/la

rge

Mi/M

oIm

paire

dplatelet

func

tion.

ITGB3(17q

21)

TUBB1-relatedthrombo

cytope

nia(TUBB1-RT,

6131

12)22

AD

TUBB1(20q

13)

1/la

rge

A/M

o—

CYCS-related

thrombo

cytope

nia(C

YCS-RTor

THC4,

6120

04)23

AD

CYCS(7p1

5)1/no

rmal,slightlyredu

ced

A—

GFI1b

-related

thrombo

cytope

nia(G

FI1b

-RT,

1879

00)5

AD

GFI1B

(9q3

4)1,1

1/la

rge

Mo/S

Impa

iredplateletfunc

tion.Red

uced

agran

ules.R

edce

llan

isoc

ytos

is.

PRKACG-related

thrombo

cytope

nia

(PRKACG-RT,

6161

76)4

AR

PRKACG

(9q2

1)111/la

rge

SIm

paire

dplatelet

func

tion.

FYB-related

thrombo

cytope

nia(FYB-RT,

na)7

AR

FYB

(5p1

3.1)

11,1

11/no

rmal,slightly

redu

ced

Mi/M

o—

SLF

N14

-related

thrombo

cytope

nia

(SLF

N14

-RT,

na)8

AD

SLF

N14

(17q

12)

1,1

1/normal,large

Mi/S

Impa

iredplatelet

func

tion.

FLI1-related

thrombo

cytope

nia(FLI1-RT,

na)24

AR

FLI1

(11p

24.3)

11/la

rge

Mi/M

oIm

paire

dplatelet

func

tion.

Giant

agran

ules.

Inhe

rited

thrombo

cytope

niafrom

mon

oallelic

THPO

mutation(na,

na)25

AD

THPO

(3q2

7.1)

1/no

rmal,slightlyincrea

sed

A—

TRPM7-relatedthrombo

cytope

nia(TRPM7-RT)

26

AD

TRPM7(15q

21.2)

1,1

1/la

rge

A/M

iAbe

rran

tdistributionof

gran

ules,inc

reased

numbe

ran

dan

arch

icorga

nizationof

microtubu

les.

Trop

omyosin4-relatedthrombo

cytope

nia

(TPM4-RT,

na)17

AD

TPM4(19p

13.1)

1/la

rge

Mi

—

AD,au

toso

mal

dominan

t;AR,au

toso

mal

rece

ssive;

CNS,ce

ntraln

ervous

system

;na

,no

tavailable;

OMIM

,on

lineMen

delianinhe

ritan

cein

man

;vW

D,vonWilleb

rand

disease;

XL,

X-linked

.*D

egrees

ofthrombo

cytope

nia:

1indica

tes.

1003

109platelets/L;

11,50

310

9to

1003

109platelets/L;

111,,

503

109platelets/L.

†Severity

ofblee

ding

tend

ency

inmos

tpa

tientsrepo

rted

forea

chform

:Aindica

tesab

sent;Mi,mild;Mo,

mod

erate;

andS,severe.

Hematology 2017 387

Table1.

(con

tinue

d)

Disea

se(abb

reviation,

OMIM

entry)

Inhe

ritan

ceGen

e(lo

cus)

Thrombo

cytope

nia*/p

latelet

size

Bleed

ing†

Add

ition

alfeatures

tothrombo

cytope

nia

Form

swith

additio

nalc

linically

releva

ntco

ngen

itald

efec

ts/syn

drom

icform

sWisko

tt-Aldric

hsynd

rome(W

AS,30

1000

)27

XL

WAS(Xp1

1)111/no

rmal,slightlyredu

ced

SSevereimmun

odeficien

cylead

ingto

early

death.

Eczem

a.Increa

sedriskof

maligna

nciesan

dau

toimmun

ity.

X-linked

thrombo

cytope

nia(XLT

,31

3900

)28

XL

WAS(Xp1

1)11,1

11/no

rmal,slightly

redu

ced

A/M

oMild

immun

odeficien

cy.Mild

tran

sien

tec

zema.

Increa

sedriskof

maligna

nciesan

dau

toimmun

ity.

Non

synd

romic

patientswith

only

thrombo

cytope

niaarede

scrib

ed.

Paris-Trousseau

thrombo

cytope

nia(TCPT,

1880

25),Jaco

bsen

synd

rome(JBS,1

4779

1)29

AD

Deletions

in11

q23

111/no

rmal,slightlyincrea

sed

Mo/S

Growth

retardation,

cogn

itive

impa

irmen

t,facialan

dskulld

ysmorph

isms,

malform

ations

ofthe

cardiovascular

system

,CNS,ga

strointestinal

appa

ratus,

kidn

ey,an

d/or

urinarytrac

t;othe

rmalform

ations.Im

paire

dplatelet

func

tion.

Giant

a-granu

lesan

dredu

cedde

nsegran

ules.

Thrombo

cytope

niamay

reso

lveover

time.

FLNA-related

thrombo

cytope

nia(FLN

A-RT,

na)30

XL

FLNA(Xq2

8)11/la

rge

Mi/M

oPeriven

tricular

nodu

larh

eterotop

ia(O

MIM

3000

49).

Non

synd

romic

patientswith

only

thrombo

cytope

niaarede

scrib

ed.

GATA

1-relateddiseases:X-linked

thrombo

cytope

niawith

thalassemia

(XLT

T,31

4050

),X-linked

thrombo

cytope

niawith

dyserythropo

ietic

anem

ia(XLT

DA,30

0367

)31

XL

GATA

1(Xp1

1)111/no

rmal,slightlyincrea

sed

Mi/S

Hem

olytic

anem

iawith

labo

ratory

abno

rmalities

resemblingb-th

alassemia,sp

leno

meg

aly,

and

dyserythropo

ietic

anem

ia.Con

genital

erythrop

oietic

porphyria

.Th

rombo

cytope

nia-ab

sent

radius

synd

rome

(TAR,27

4000

)32

AR

RBM8A

(1q2

1)111/no

rmal,slightlyredu

ced

SBilateralrad

iala

plasia

1/2

additiona

lupp

eran

dlower

limbab

norm

alities.Pos

siblekidn

ey,

cardiac,

and/or

CNSmalform

ations.Pos

sible

intoleranc

eto

cow’s

milk

that

canbe

asso

ciated

with

exac

erba

tionof

thrombo

cytope

nia.

Red

uced

meg

akaryocytesin

bone

marrow.Plateletco

unt

may

riseover

time.

Storm

orkensynd

rome(STR

MK,185

070)

6AD

STIM1(11p

15)

11,1

11/na

Mi

Myopa

thywith

tubu

larag

greg

ates,co

ngen

ital

miosis,

func

tiona

loran

atom

ical

asplen

ia,

ichthyos

is,he

adac

he,mild

anem

ia,facial

dysm

orph

isms,

defectsin

physical

grow

th,an

dco

gnitive

impa

irmen

t.Yorkplatelet

synd

rome(YPS,na

)33

AD

STIM1(11p

15)

11,1

11/normal

AMyopa

thy,

platelet

ultrastruc

turala

bnormalities,

such

asmod

eratelyde

crea

sedagran

ules,

increa

sedvacu

oles,and

gian

telectronde

nsean

dtargeted

-like

bodies.

AD,au

toso

mal

dominan

t;AR,au

toso

mal

rece

ssive;

CNS,ce

ntraln

ervous

system

;na

,no

tavailable;

OMIM

,on

lineMen

delianinhe

ritan

cein

man

;vW

D,vonWilleb

rand

disease;

XL,

X-linked

.*D

egrees

ofthrombo

cytope

nia:

1indica

tes.

1003

109platelets/L;

11,50

310

9to

1003

109platelets/L;

111,,

503

109platelets/L.

†Severity

ofblee

ding

tend

ency

inmos

tpa

tientsrepo

rted

forea

chform

:Aindica

tesab

sent;Mi,mild;Mo,

mod

erate;

andS,severe.


Table1.

(con

tinue

d)

Disea

se(abb

reviation,

OMIM

entry)

Inhe

ritan

ceGen

e(lo

cus)

Thrombo

cytope

nia*/p

latelet

size

Bleed

ing†

Add

ition

alfeatures

tothrombo

cytope

nia

Form

swith

increa

sedris

kof

acqu

iring

additio

nalilln

esse

s/pred

ispo

sing

form

sMYH9-relateddisease(M

YH9-RD

(na)

14

AD

MYH9(22q

12)

1,1

11/giant,large

A/M

iPos

siblede

velopm

entof

sensorineu

rald

eafness,

neph

ropa

thy,an

d/or

cataract.H

alfo

fthe

patients

presen

twith

elevated

liver

enzymes

withou

tde

veloping

liver

dysfun

ction.

DIAPH1-relatedthrombo

cytope

nia

( DIAPH1-RT,

na)10

AD

DIAPH1(5q3

1.3)

1,1

11/la

rge

ARiskof

sensorineu

rald

eafnessdu

ringinfanc

y.Pos

siblemild

tran

sien

tleukop

enia.

Con

genitala

meg

akaryocytic

thrombo

cytope

nia

(CAMT,

6044

98)34

AR

MPL(1p3

4.2)

111/normal,slightlyredu

ced

SRed

uced

/abs

entmeg

akaryocytes.

Evolutionto

severe

bone

marrow

aplasiain

infanc

yin

all

patients.

Rad

ioulna

rsyno

stos

iswith

ameg

akaryocytic

thrombo

cytope

nia(RUSAT,

6054

32,

6167

38)35,36

AD/AR

HOXA11

(7p1

5)or

MECOM

(3q2

6.2)

111/no

rmal,slightlyincrea

sed

SBilateralrad

io-ulnar

syno

stos

is1/2

othe

rmalform

ations.R

educ

ed/abs

entm

egakaryocytes

inbo

nemarrow.Pos

sibleprog

ressionto

bone

marrow

aplasia.

Thehe

matolog

ical

phen

otypeis

moresevere

intheARform

beca

useof

MECOM

mutations.

Familialplatelet

diso

rder

with

prop

ensityto

acute

myeloge

nous

leukem

ia(FPD/AML,

6013

99)37

AD

RUNX1(21q

22)

11/no

rmal,slightlyincrea

sed

A/M

oOver40

%of

patientsac

quire

acutemyeloge

nous

leukem

iaor

myelodysp

lastic

synd

romes.

Increa

sedriskof

Tac

utelymph

oblasticleukem

ia.

Impa

iredplatelet

func

tion.

ANKRD26

-related

thrombo

cytope

nia

(ANKRD26

-RT(orTH

C2,

1880

00)16

AD

ANKRD26

(10p

12)

11,1

11/no

rmal,slightly

increa

sed

A/M

iAbo

ut8%

ofpa

tientsac

quire

myeloidmaligna

ncies.

Som

epa

tientsha

veincrea

sedlevels

ofhe

mog

lobinan

d/or

leukoc

ytes.Red

uced

agran

ules

inso

mepa

tients.

ETV

6-relatedthrombo

cytope

nia(ETV

6-RT,

na)9

AD

ETV

6(12p

13)

1,1

1/no

rmal,slightlyincrea

sed

A/M

oAbo

ut25

%of

patientsac

quire

acutelymph

oblastic

leukem

iaan

dothe

rhe

matolog

ical

maligna

ncies.

SRC-related

thrombo

cytope

nia(SRC-RT)

11

AD

SRC

(20q

11.23)

11,1

11/la

rge

Mo/S

Con

genitalfac

iald

ysmorph

ism,juvenile

myelofibros

isan

dsp

leno

meg

aly,

severe

osteop

oros

is,prem

atureed

entulism.Platelets

hypo

gran

ular

orag

ranu

lar.Abu

ndan

tvacu

oles.

AD,au

toso

mal

dominan

t;AR,au

toso

mal

rece

ssive;

CNS,ce

ntraln

ervous

system

;na

,no

tavailable;

OMIM

,on

lineMen

delianinhe

ritan

cein

man

;vW

D,vonWilleb

rand

disease;

XL,

X-linked

.*D

egrees

ofthrombo

cytope

nia:

1indica

tes.

1003

109platelets/L;

11,50

310

9to

1003

109platelets/L;

111,,

503

109platelets/L.

†Severity

ofblee

ding

tend

ency

inmos

tpa

tientsrepo

rted

forea

chform

:Aindica

tesab

sent;Mi,mild;Mo,

mod

erate;

andS,severe.

Hematology 2017 389

Table 2. Main pathogenetic mechanisms of inherited thrombocytopenias

Disease GeneFunction of the defective gene/pathogenetic

mechanisms of thrombocytopenia

Defects in megakaryocyte differentiationCongenital amegakaryocytic thrombocytopenia39 MPL MPL encodes for the receptor for THPO. MPL biallelic

mutations abrogate THPO-MPL signaling, which isessential for the commitment differentiation ofmultipotent hematopoietic stem cells to MKs since birthand the self-renewal and maintenance of the multipotentstem cell compartment in postnatal hematopoiesis.

Thrombocytopenia-absent radius syndrome32 RBM8A RBM8A encodes for Y14, a component of the ubiquitousexon-junction complex, which is involved in RNAprocessing. Thrombocytopenia-absent radius syndromeis caused by compound inheritance of a low-frequencynoncoding SNP and a rare null allele in RBM8A thatsignificantly reduces Y14 expression. It has beenhypothesized that Y14 deficiency affects MKdifferentiation by impairing the signaling downstreamMPL.

Radioulnar synostosis with amegakaryocyticthrombocytopenia36

HOXA11 HOXA11 encodes for a member of the Homeobox family ofDNA-binding proteins involved in the regulation of earlyhematopoiesis. Mutation of HOXA11 responsible forRUSAT affects MK differentiation in vitro.

MECOM MECOM encodes the oncoprotein EVI1, a transcriptionfactor involved in homeostasis of the hematopoietic stemcell compartment and MK differentiation. Missensepathogenetic variants of EVI1 may reduce its interactionwith DNA and/or other transcription factors.

Defects in megakaryocyte maturationFamilial platelet disorder with propensity to acutemyelogenous leukemia40

RUNX1 (AML1, CBFA2) RUNX1 encodes for the DNA-binding a subunit of theCBFtranscription complex, which transactivates multiplehematopoiesis-specific genes. Pathogenetic variantsinduce profound defects in MK maturation and plateletfunctional abnormalities deriving from dysregulatedexpression of several genes involved in MKdevelopment. Moreover, they affect the clonogenicpotential and self-renewal capacities of thehematopoietic myeloid progenitors.

ANKRD26-related thrombocytopenia41 ANKRD26 ANKRD26 is downregulated during MK maturation bybinding of RUNX1 and FLI1 to the 59 UTR of the gene.Pathogenetic mutations abolish this binding, resulting inANKRD26 overexpression in MKs, which, in turn,induces unbalanced activation of kinases downstreamthe MPL receptor, especially the MAPK/ERK 1/2pathway. This mechanism induces altered MKmaturation and reduced proplatelet extension.

Paris-Trousseau thrombocytopenia/Jacobsensyndrome29

Deletions in 11q23 Contiguous gene deletion syndrome. Different sizes andbreakpoints of deletions are responsible forheterogeneity of the clinical picture. Thrombocytopeniaderives from the deletion of FLI1, a transcription factorthat promotes platelet production by transactivation ofseveral genes associated with MK development, suchas MPL, GP6, GP9, GP1BA, ITGA2B, and PF4. FLI1haploinsufficiency results in altered MK maturation anddysmorphic and dysfunctional platelets.

FLI1-related thrombocytopenia24 FLI1 A homozygous missense mutation of FLI1 (see above)results in impaired DNA binding of the transcriptionfactor and causes a phenotype similar to that of Paris-Trousseau thrombocytopenia.

CBF, core binding factor; ERK, extracellular signal-regulated kinase; Mk, megakaryocyte. Additional abbreviations are explained in Table 1.


Table 2. (continued)



ETV6-related thrombocytopenia9 ETV6 ETV6 encodes an ETS transcription factor that was initiallyidentified as a tumor suppressor. ETV6 is involved in thebalance between proliferation and differentiation of earlyhematopoietic progenitors and in late MK development.Mutations cause thrombocytopenia by affecting MKmaturation.

GATA1-related diseases42 GATA1 GATA1 is a transcription factor regulating several geneswith a key role in megakaryopoiesis, as GP1BA,GP1BB, ITGA2B, PF4, MPL, and NFE2, and inerythropoiesis, as HBB, ALAS1, and BCL2L1.Mutations affect MK maturation, resulting inthrombocytopenia and the production of dysmorphicand dysfunctional platelets.

GFI1b-related thrombocytopenia5 GFI1B GFI1B is a transcription factor regulating homeostasis ofhematopoietic stem cells and development of the MKand erythroid lineages. Mutations cause defective MKmaturation with defective expression of several plateletproteins, a granule deficiency, and variable alterations ofplatelet function.

Gray platelet syndrome43 NBEAL2 NBEAL2 encodes neurobeachin-like protein 2, which isinvolved in protein-protein interactions, membranedynamics, and vesicle trafficking. Mutations result indefective MK maturation, severe deficiency of plateleta granules, and variable defects of platelet function.

SLFN14-related thrombocytopenia8 SLFN14 The role of SLFN14 protein is poorly known. Possible roleas an endoribonuclease, regulating rRNA, andribosome-associated mRNA cleavage. Mutationsinduce reduced SLFN14 expression in platelets.Patients’ platelets have defective dense granuleformation and ATP secretion and heterogeneousfunctional defects. Some observations suggested thatthrombocytopenia derives from reduced MK maturationand proplatelet formation.

FYB-related thrombocytopenia7 FYB The FYB protein (ADAP) is a candidate linker between cellmembrane activation signals and intracellular eventsregulating actin organization. Because of immature MKsin bone marrow and activated platelets in blood, it hasbeen hypothesized that thrombocytopenia derives fromdefective maturation of MKs and clearance of activatedplatelets.

SRC-related thrombocytopenia11 SRC SRC encodes for a tyrosine kinase, with a key rolein several MK and platelet signaling pathways.Thrombocytopenia is caused by a gain-of-functionmissense mutation, resulting in a constitutive kinaseactivation, which causes increased overall tyrosinephosphorylation. Some observations suggest thatthrombocytopenia derives from defective MKmaturationand reduced proplatelet formation.

Defects in platelet releaseMYH9-related disease43 MYH9 The MYH9 gene encodes for the heavy chain of

non-muscle myosin IIA, a cytoplasmic myosininvolved in processes requiring the generation ofa chemomechanical force by the actin cytoskeleton.Mutations cause macrothrombocytopenia by inducingmultiple defects of proplatelet formation and possiblepremature, ectopic release of platelets within the bonemarrow.

ACTN1-related thrombocytopenia19 ACTN1 The ACTN1 protein (a1 actinin) stabilizes the actincytoskeleton by crosslinking actin filaments in bundles.Mutations cause macrothrombocytopenia by inducingmultiple defects of proplatelet formation.


Hematology 2017 391




FLNA-related thrombocytopenia30 FLNA The FLNA protein (filamin A) stabilizes the actincytoskeleton and connects it to the plasma membrane.In MKs, filamin A binds the intracytoplasmatic domainof GPIba to the actin filament network. It has beensuggested that mutations result in defective proplateletformation.

Bernard-Soulier syndromeBiallelic43 GP1BA GPIBB GP9 These genes encode for components of the GPIb-IX-V

complex in platelets and MKs. Mutations result indefective expression of the complex in the plasmamembrane and cause thrombocytopenia by hamperingproplatelet formation.

Monoallelic43

ITGA2B/ITGB3-related thrombocytopenia44 ITGA2B ITGB3 ITGA2B and ITGB3 encode for the components ofthe GPIIb-IIIa complex. Mutations responsible formacrothrombocytopenia cause constitutive activationof the complex, which affects actin cytoskeletonreorganization. Proplatelet formation and proplateletconversion to platelets are defective.

TUBB1-related thrombocytopenia22 TUBB1 The TUBB1 protein (b1 tubulin) is a component ofmicrotubules expressed only in mature MKs andplatelets. Mutations disrupt microtubule assemblyand result in defective proplatelet formation.

TRPM7-related thrombocytopenia26 TRPM7 TRPM7 is a cation channel regulating calcium andmagnesium homeostasis and a kinase that modulatesthe activity of non-muscle myosin IIA. Mutations inducemacrothrombocytopenia deriving from altered acto-myosin cytoskeletal reorganization, which, in turn,impairs proplatelet formation.

TPM4-related thrombocytopenia17 TPM4 Tropomyosins are cytoskeletal proteins that regulatemultiple functions of the acto-myosin cytoskeleton.TPM4 encodes 2 tropomyosin isoforms that areabundantly expressed in MKs and platelets.Mutations cause TPM4 insufficiency, which inducesmacrothrombocytopenia by altering actin cytoskeletonreorganization and eventually proplatelet formation.

CYCS-related thrombocytopenia45 CYCS CYCS encodes cytochrome c, a ubiquitous mitochondrialprotein involved in mitochondrial respiration and initiationof the intrinsic pathway of apoptosis. Data suggest thatmutations cause thrombocytopenia by inducing ectopic,premature release of platelets by a proplatelet-independent mechanism.

DIAPH1-related thrombocytopenia10 DIAPH1 DIAPH1 encodes a conserved member of the forminprotein family, which mediates r GTPase-dependentassembly of filamentous actin and microtubuleregulation during cytoskeletal remodeling. Mutationcauses constitutive activation of the protein, whichaffects proplatelet formation by inducing abnormalitiesof cytoskeletal and microtubule function.

PRKACG-related thrombocytopenia4 PRKACG PRKACG encodes the g isoform of the catalytic subunit ofcAMP-dependent protein kinase A. Mutations affectproplatelet formation and platelet activation.

Shortened platelet survivalPlatelet-type von Willebrand disease20 GP1BA The gene GP1BA encodes GPIba, whose extracellular

domain binds to vWF. The disorder derives from gain-of-function mutations that increase the affinity of GPIba forvon Willebrand factor. Reduced platelet survival derivesfrom platelet clumping within the circulation.



GPIb/IX/V and GPIIb/IIIa, ie, biallelic and monoallelic BSS andITGA2B/ITGB3-related thrombocytopenia (ITGA2B/ITGB3-RT).Macrothrombocytopenia results from disruption of the interactions ofthese membrane integrins with the actomyosin cytoskeleton, whichare essential for preserving MK cytoskeletal structure and re-organization.43 For instance, mutations responsible for ITGA2B/ITGB3-RT are gain-of-function variants resulting in constitutive, in-appropriate activation of the GPIIb/IIIa. Thrombocytopenia of thisdisorder is explained by defects in proplatelet formation and con-version of proplatelets into platelets. Recent observations showed howthese defects actually derive from altered remodeling of the actincytoskeleton, which, in turn, is caused by hyper-activation of theoutside-in signaling downstream of GPIIb/IIIa.44

Clinical pictureBleeding diathesisThe presence and severity of bleeding tendency is greatly variableamong patients with IT. Some patients present with no bleeding atall, whereas others may have hemorrhages only during hemo-static challenges. A minority of patients presents with spontaneousbleeding and may even develop life-threatening bleeding episodes.Bleeding is muco-cutaneous and consists of petechiae, ecchymoses,epistaxis, menorrhagia, and gastrointestinal hemorrhages. The de-gree of bleeding tendency correlates with platelet count in thoseITs that are not associated with significant platelet dysfunction.14

Conversely, bleeding tendency is usually more severe than ex-pected on the basis of platelet count in those forms associated withalteration of platelet function. For instance, the degree of bleedingdiathesis is independent of platelet count in patients with biallelicBSS, suggesting that the impaired interaction between plateletsand the subendothelium due to the GPIb/IX/V defect is the majordeterminant of bleeding.18 Clinically relevant defects of plateletfunction are variably present in patients with Gray platelet syndrome(GPS), GFI1b-related thrombocytopenia, SLFN14-related throm-bocytopenia, PRKACG-related thrombocytopenia, ITGA2B/ITGB3-RT, Paris-Trousseau thrombocytopenia/Jacobsen syndrome,FPD/AML, and Wiskott-Aldrich syndrome (WAS) (Table 1).

The degree of thrombocytopenia remains stable during life in thelarge majority of ITs. However, platelet count tends to increaseover time in thrombocytopenia-absent radius syndrome and insome patients with Paris-Trousseau thrombocytopenia/Jacobsensyndrome.29 Conversely, thrombocytopenia worsens over the yearsin GPS,3 whereas in patients with platelet-type von Willebrand dis-ease, the platelet count may be variable and typically decreases understressful conditions.20

Syndromic formsIn some patients, the molecular defects responsible for thrombocy-topenia induce complex syndromic phenotypes. Congenital defectsassociated with thrombocytopenia in these ITs include skeletal de-formity, cognitive impairment, malformations of the central nervous orcardiovascular system, and immunodeficiency. The abnormalitiestypical of the single syndromic forms are listed in Table 1.

Predisposing formsThe increasing number of ITs identified over the last few years let usunderstand that bleeding is no longer the only clinical risk for patientswith IT. In fact, the development of hematological malignancies, bonemarrow aplasia, juvenile myelofibrosis, or end-stage renal disease canbe far more dangerous for the patient’s life than hemorrhages. Themain predisposing forms are briefly discussed below.

MYH9-RD. With more than 300 families reported, MYH9-relateddisease (MYH9-RD) is the most prevalent IT worldwide. It is causedby monoallelic mutations in MYH9, the gene for the heavy chainof non-muscle myosin IIA (NMMHC-IIA). All patients present atbirth with giant platelets and thrombocytopenia, which may resultin bleeding tendency of different degrees. In some cases, macro-thrombocytopenia remains the only manifestation of the diseasethroughout life; however, about one-third of patients with MYH9-RDdevelop proteinuric nephropathy, often leading to end-stage renaldisease. Moreover, most individuals with MYH9-RD acquire senso-rineural hearing loss, ranging from a mild defect to profound deafness,and about 20% of patients develop presenile cataracts.14,49 Theinvestigation of a wide series of consecutive patients identified




Wiskott-Aldrich syndrome27 WAS The WAS protein plays a key role in the polymerizationof actin in hematopoietic cells. Mutations resultin increased clearance of platelets by thereticuloendothelial system and premature, ectopicplatelet release of platelets in the bone marrow. Theobservation that splenectomy normalizes or significantlyincreases platelet counts in patients with WAS/XLTsuggests that increased platelet clearance is the keymechanism of thrombocytopenia.

X-linked thrombocytopenia27

Unknown defectStormorken syndrome/York platelet syndrome6,35 STIM1 STIM1 encodes for a protein of the endoplasmic reticulum

that regulates Ca21 influx to cells through the Ca21

release-activated Ca21 channels of the plasmamembrane in many cell types. The causative variants aregain-of-function monoallelic mutations, resulting inconstitutive Ca21 influx from the extracellular space tothe cytoplasm. Platelets have several in vitro defects ofaggregation, activation, and d granule secretion.


Hematology 2017 393

genotype-phenotype correlations that allow the prediction of theevolution of the disease in about 85% ofMYH9-RD cases. In general,mutations affecting the N-terminal head domain of NMMHC-IIA areassociated with a worse prognosis than those in the C-terminal taildomain. Moreover, a recent study showed how different mutationswithin the same NMMHC-IIA domain, and even hitting the sameresidue, may be associated with a significantly different risk of extra-hematological manifestations, thus providing a more accurate prog-nostic model.14

CAMT and radioulnar synostosis with amegakaryocyticthrombocytopenia. Due to biallelicmutations inMPL, the gene for thereceptor of thrombopoietin (THPO), CAMT presents at birth as isolatedhypomegakaryocytic thrombocytopenia without other phenotypic ab-normalities. Patients with CAMT develop further cytopenias duringchildhood until progression to generalized bone marrow aplasia.34

Radioulnar synostosis with amegakaryocytic thrombocytopenia isa genetically heterogeneous disorder that can be caused by mutationsin HOXA11 or MECOM. Development of bone marrow aplasia hasbeen reported in both variants of the disease, even if patients withMECOM mutations seems to have a more severe evolution.36

FPD/AML, ANKRD26-RT, and ETV6-RT. These 3 autosomal-dominant disorders, with a quite different prevalence among ITs(3%, 18%, and 5%, respectively), share several clinical features. Allof them present with isolated, non-syndromic thrombocytopenia,which is usually mild to moderate, with some patients havinga platelet count above the lower limit of the normal range.16,37 Unlikethe majority of ITs characterized by variable degrees of plateletmacrocytosis, platelet size is normal; morphology of blood cellsis also preserved. Bleeding is often absent or mild, especially inpatients with ANKRD26-related thrombocytopenia (ANKRD26-RT)or ETV6-related thrombocytopenia (ETV6-RT). Patients with FPD/AML may present with variable degrees of bleeding tendency dueto heterogeneous abnormalities of platelet function.37 Finally, butmost importantly, another common feature of these conditions is theincreased risk of hematological malignancies. Myelodysplasticsyndromes and AML are reported in about 40% of patients withFPD/AML and 8% of patients with ANKRD26-RT. Of the 73 patientswith ETV6-RT described so far, 17 (23%) had hematological ma-lignancies: 7 patients developed acute lymphoblastic leukemia,whereas the remaining 10 had AML, myelodysplastic syndromes,multiple myeloma, or polycythemia vera.9,50 Of note, the “2016revision of the World Health Organization classification of myeloidneoplasms and acute leukemia” included the myeloid malignanciesarising from germline mutations in ANKRD26, RUNX1, and ETV6in a new category defined as “Myeloid neoplasms with germlinepredisposition and pre-existing platelet disorders.”51

SRC-Related Thrombocytopenia. A dominant gain-of-functionmutation of SRC has been recently described as causative of IT ina large pedigree. Platelets are dysmorphic and highly variable in size,with paucity of a granules. Five out of 9 affected individuals developedearly-onset myelofibrosis, with hypercellular bonemarrow and trilineagedysplasia. With the limitation of the observation of a single family, thisIT should be regarded as a possible cause of juvenile myelofibrosis.11

DiagnosisThere are 3 main questions when dealing with the diagnosis of ITs:how to recognize that the thrombocytopenia we are investigating is

a genetic form; how to reach a molecular diagnosis of a supposed IT;and which are the forms that require a molecular diagnosis. Aneffective diagnostic pathway cannot disregard the scrupulous col-lection of personal and family history, careful physical examination,and analysis of peripheral blood smears.

How to recognize that the thrombocytopenia we areinvestigating is a hereditary formA diagnosis of IT may be straightforward when an informative familyhistory is available or the low platelet count has been documentedsince birth. However, recognition of the genetic origin of thrombo-cytopenia is often difficult, as demonstrated by the fact that manypatients with IT are misdiagnosed with acquired thrombocytopenias.For instance, the recent investigation of a cohort of 181 women withITs showed that 31% of them were misdiagnosed with ITP; for thisreason, 25% received 1 or more lines of undue immunosuppressivetherapy, and 8% underwent unnecessary splenectomy.52

The genetic origin of a thrombocytopenia may not appear imme-diately evident because of several reasons: (1) many ITs are recessiveforms (Table 1); (2) a number of patients carry de novo mutations, asdocumented in MYH9-RD, in which up to 40% of subjects havesporadic forms due to de novo mutational events14; (3) the pene-trance of the causative mutation may be incomplete, as reported inmonoallelic BSS, ANKRD26-RT, tropomyosin 4–related thrombo-cytopenia, and FPD/AML16-18,37; and (4) thrombocytopenia is oftendiscovered only in adulthood because of the mild or absent bleedingtendency.53 Some helpful suggestions may be derived from the basicexamination of patients. A lifelong history of bleeding, a bleedingtendency more severe than expected based on the platelet count, thepresence of manifestations typically associated with thrombocyto-penia in syndromic forms, and the finding of giant or dysmorphicplatelets at the peripheral blood smear examination make the di-agnosis of a genetic form more likely. For all the critical aspectsdiscussed above, we recommend consideration of an inherited formwhenever the acquired origin of thrombocytopenia is not obvious.

How to reach a molecular diagnosis of a supposed ITThe recent introduction of HTS technologies made it possible tomanage the diagnostic workup of ITs either by the traditional tieredapproach or by the quicker single-step approach. Until a few years ago,once the genetic nature of a thrombocytopenia was defined, a series oflaboratory tests (in vitro platelet aggregation,flow cytometry for plateletsurface GPs, examination of peripheral blood smear, and immuno-fluorescence assay for MYH9 protein aggregates in neutrophils) wereperformed to identify the candidate gene/genes to be sequenced. Thisapproach was the mainstay of the diagnostic algorithm proposed in2003 by the Italian Platelet Study Group and subsequently validatedin different centers and progressively updated.13,54

Nowadays, targeted NGS platforms can be efficiently applied tosearch for the causative gene.2 Because the molecular basis of thedisease remains unknown in a relevant proportion of patients with IT,despite the application of extensive targeted HTS studies, WES orWGS may be required. However, a positive finding is not alwayspossible. In fact, a number of changes in many genes are often foundand distinguishing the pathogenetic variants from nonpathogeneticvariants may require complex functional studies.55

Although at the moment, the diagnostic workup is also largelydependent on local availability in terms of technical skills, expertise,


and funds, it is likely that targeted NGS platforms will soon becomethe routine diagnostic test for ITs.

Which ITs require a molecular diagnosisDespite the increasing accessibility to NGS, a rational and cost-effective diagnostic approach to ITs is requested. However, wesuggest that a definitive diagnosis must always be sought after, atleast in subjects in whom a predisposing form is suspected (Table 1).Before doing this, patients with a possible diagnosis of FPD/AML,ANKRD26-RT, and ETV6-RT should be exhaustively informedabout the impossibility to predict whether and when they willpossibly develop a hematological neoplasm and that no means arecurrently available to prevent such an eventuality. Figure 2 proposesa diagnostic algorithm for ITs predisposing to additional illnessesbased on the evaluation of a few basic patients’ clinical features.

After excluding a syndromic IT by physical examination, evaluationof platelet size on peripheral blood smears can guide the diagnosticworkup.12,14 In patients with large platelets, the suspicion forMYH9-RD should be raised independently of the presence of additionalclinical features of the disease.14 In patients with normal MYH9distribution in neutrophils, DIAPH1-related thrombocytopenia andSRC-related thrombocytopenia should be considered. In children andyoung individuals with thrombocytopenia and normal-sized plate-lets, with or without anemia and/or neutropenia, a bone marrow ex-amination should be performed to search for CAMT; very high serumthrombopoietin levels will support this diagnostic hypothesis whilenot being sufficient. In children in whom CAMT is excluded and inadults with normal platelet size, FPD/AML, ANKRD26-RT, andETV6-RT should be considered, especially if family history is con-sistent with autosomal dominant transmission. Molecular analysis isrequired to confirm the diagnosis and provide patients with person-alized management, counseling, and follow-up.

Follow-upA close follow-up is essential for patients with predisposing forms,although medical attention must be provided to all patients with IT in

case of bleeding or for its prevention during hemostatic challenges.Follow-up of patients withMYH9-RD should be tailored on the basisof the specific NMMHC-IIA mutation and should include periodicmeasurements of proteinuria, which is the earliest sign of kidneyinvolvement.14 In fact, patients with early-stage kidney disease maybenefit from treatment of reducing proteinuria.56 Periodic evaluationof renal function and the search for sensorineural hearing loss andcataract are also recommended.

In patients with a diagnosis of FPD/AML, ANKRD26-RT, and ETV6-RT, a baseline bone marrow examination, including cytogeneticanalysis, should be considered. Thereafter, patients should be moni-tored by performing a complete blood count and peripheral bloodsmear examination once a year. In the case of development of lab-oratory data (ie, appearance of leukocytosis, abnormal white blood celldifferential, unexplained anemia, and worsening of thrombocytopenia)and/or symptoms suggestive of hematological malignancies, patientsshould be further investigated by bone marrow examination and otherappropriate studies.

In patients with a predisposition to hematological neoplasms, HLAtyping and the search for a compatible donor for hematopoietic stemcell transplantation (HSCT) before the possible occurrence of an overtneoplasm may be discussed with each single patient. Of note, thefamily members should be investigated at the molecular level to avoidthe use of a relative affected by the same IT as the donor for HSCT.16,57

TherapyMost patients with ITs have no or mild spontaneous bleeding andrequire medical surveillance and sometimes prophylactic interventiononly on the occasion of hemostatic challenges, such as surgery, otherinvasive procedures, or deliveries. Treatment of hemorrhages, whichare usually mucocutaneous, is mainly based on local measures andplatelet transfusions. Some agents for improving hemostasis are alsowidely used in clinical practice and recommended by availableguidelines,58 even if evidence on their clinical effectiveness is mostlyanecdotal. Table 3 recapitulates the currently available measures forthe treatment of ITs, and Figure 3 outlines the approach we use forthe management of bleeding episodes.

When managing patients affected by ITs without significant plateletdysfunction, the indication for the use of prophylactic plateletsprior to elective invasive procedures can be deduced from generalguidelines for platelet transfusion.59,60 On the contrary, patientsaffected by ITs with clinically relevant platelet dysfunction may needprophylactic intervention even for platelet counts (, 503 109/L fordelivery and , 68 3 109/L for surgery) higher than the thresholdsrecommended for “safe” surgery. Two recent studies provided asystematic assessment of the risk of bleeding associated with de-livery and surgery,52,61 respectively, by the investigation of a largeseries of patients with IT. Patients’ previous history of bleeding andlow platelet count were the major predictors of excessive bleedingboth postpartum and after surgery. Age over 70 years and diagnosisof some ITs with concurrent platelet dysfunction, ie, biallelic BSS,FPD/AML, GPS, and ITGA2B/ITGB3-RT, were also associ-ated with an increased risk of excessive surgical bleeding.61 In-terestingly, in both studies, prophylactic platelet transfusion was notassociated with a reduced incidence of peri-procedural hemor-rhages; however, transfusions were given to patients with a higherbleeding risk, making it difficult to interpret these results. Indeed,the fact that the incidence of peri-procedural bleeding was not in-creased in the patients with a higher risk of bleeding suggests that

Figure 2. Diagnostic algorithm for hereditary thrombocytopeniaspredisposing to additional illnesses based on the evaluation of a few basicpatients’ clinical features. Abbreviations are explained in Table 1.

Hematology 2017 395

prophylactic platelet transfusions were effective in reducing the rateof hemorrhages.

THPO-receptor agonists opened new prospects in the treatment ofITs because they provided a potential pharmacological option toincrease platelet count of these patients for the first time. In a phase 2clinical trial, eltrombopag was given for 3 to 6 weeks to 12 patientswith MYH9-RD and a platelet count below 50 3 109/L. A total of11 patients achieved significant platelet response and remission ofspontaneous bleeding without relevant adverse events.62 On the basisof these findings, short-term eltrombopag was successfully usedfor preparing patients with MYH9-RD and severe thrombocytopeniafor major surgery.63,64 Elective surgery prepared by short-termeltrombopag administration was also reported in 1 subject withANKRD26-RT.65 More recently, a group from the United Statesinvestigated the effects of long-term administration of eltrombopag(22 weeks to 4 years) in 8 patients with WAS/X-linked thrombo-cytopenia and chronic or recurrent spontaneous bleeding. Five

obtained a platelet response and durable remission or reductionof spontaneous bleeding and no major adverse events.66,67 Thus,eltrombopag represents an option for the control of bleeding in patientswithWAS or X-linked thrombocytopenia who are not candidates to orwaiting for HSCT. Differently from splenectomy (the other availableoption; see Table 3), eltrombopag is not expected to worsen theimmunodeficiency typical of the diseases. Of note, the effect of long-term eltrombopag on the outcome of HSCT is still to be determined. Ofcourse, THPO-receptor agonists may be proposed only in the forms ofHT, in which platelet function is normal, or only mildly reduced, sothat endogenous platelets can efficiently promote hemostasis.

HSCT is the treatment of choice in WAS and CAMT, and may beconsidered in selected patients affected with other ITs and presentingwith severe clinical manifestations and/or poor prognosis.36,68,69 Themedian life expectancy of patients withWASwho are not transplantedis approximately 15 years, whereas HSCT can cure all the features ofthe disease. The 5-year overall survival of patients who underwent

Table 3. Currently available measures for treatment of inherited thrombocytopenias

Treatment Indications

Tools for stopping bleeding and covering hemostatic challengesLocal measures (eg, nasal packing or cauterization, suturing) Whenever possible, should be considered as the first-line treatment of bleeding

unless it is life or organ threatening or occurs at functionally critical sites.Platelet transfusions Currently considered as the standard measure to stop bleeding or cover

hemostatic challenges. Their use should be limited to clinical situations thatcannot be managed otherwise, mainly because of the risk of HLAalloimmunization that can lead to refractoriness to subsequent plateletinfusions. Use HLA-matched concentrates whenever possible. Should beconsidered as the first-line treatment of life- or organ-threatening bleeding orbleeding at critical sites.

Antifibrinolytic agents Widely used in clinical practice to treat bleeding or cover hemostatic challenges(especially low-risk procedures).

Anecdotal evidence about clinical efficacy in ITs; use based on experts’ opinion.Desmopressin Widely used in clinical practice to treat bleeding or cover hemostatic challenges

(especially low-risk procedures).Anecdotal evidence about clinical efficacy in ITs; use based on improvement of

bleeding time in some forms.Recombinant factor VIIa Limited clinical experience in ITs; it has been used to treat bleeding or cover

surgery in few patients with bBSS (case reports).Short-term eltrombopag Effective in increasing platelet count in most patients with MYH9-RD (phase 2

trial).It has been used to cover elective surgery in few patients with MYH9-RD or

ANKRD26-RT (case reports).

Tools for achieving long-term increase of platelet countLong-term eltrombopag Tested in 8 patients with WAS/XLT (phase 2 trial). Five of them achieved

a clinical response. No major side effects were recorded.Splenectomy Increases platelet count in patients with WAS/XLT but also increases the

incidence of infections, without affecting overall survival. Splenectomy has norole in all the other forms of IT.

In XLT, the pros and cons of splenectomy should be assessed in each individualpatient.

In WAS, splenectomy should be avoided in candidates for HSCT because itincreases the risk of major infections after HSCT.

HSCT Treatment of choice for WAS and CAMT.Should be considered in selected patients affected with other ITs presenting with

severe clinical manifestations and/or poor prognosis.It has been used in a few patients with XLT, RUSAT, bBSS, and

thrombocytopenia-absent radius syndrome, with good outcomes.Gene therapy Experimental option for patients affected by severe WAS who do not have

a suitable donor for HSCT.

Abbreviations are explained in Table 1.


HSCT after the year 2000was 100% for those transplanted fromHLA-matched related siblings and 90% for HLA-matched unrelatedor mismatched-related donor procedures.70,71 The outcome of HSCTremarkably improved over the last few years for both patients trans-planted from mismatched family donors and those treated at age olderthan 5 years using an unrelated donor. Among the 63 patients withCAMT registered in the European Society for Blood and MarrowTransplantation database in the years 1987 to 2013, 30were transplantedfrom matched family donors, 25 were transplanted from matchedunrelated donors, and 8 were transplanted from mismatched donors.The 5-year overall survival was 77%, and transplant-related mortalitywas 12.6%, with no clear differences by age, year of transplant, HLAcompatibility, or stem cell source.72 Gene therapy for correction ofWASp protein deficiency represents an experimental option for pa-tients affected byWASwho do not have a suitable donor for HSCT. Inthe first gene therapy clinical trial, treatment was complicated bysevere vector-related genotoxicity.73 Two subsequent trials treated 11patients using a second-generation lentiviral vector.67,74,75 Ten patientsachieved good responses in terms of improvement of bleeding ten-dency and reconstitution of immune system functions, whereas 1 pa-tient died from a preexisting infection. No evidence of treatment-relatedgenotoxicity was found after a median follow-up of 30 months. Severaltrials of gene therapy in WAS are currently ongoing worldwide.

SummaryOver the last few years, the concept of IT has changed significantly.The major concern for some patients with IT is no longer bleedingbut additional disorders that may develop during life and be haz-ardous for the patient’s life. A multidisciplinary approach may berequired to reach the correct diagnosis of these predisposing formsand arrange the most appropriate treatment and follow-up.

CorrespondencePatrizia Noris, Department of Internal Medicine, Fondazione IRCCSPoliclinico San Matteo, Piazzale Golgi, 27100 Pavia, Italy; e-mail:[email protected].

References1. Balduini CL, Pecci A, Noris P. Inherited thrombocytopenias: the

evolving spectrum. Hamostaseologie. 2012;32(4):259-270.

2. Simeoni I, Stephens JC, Hu F, et al. A high-throughput sequencing testfor diagnosing inherited bleeding, thrombotic, and platelet disorders.Blood. 2016;127(23):2791-2803.

3. Kahr WH, Hinckley J, Li L, et al. Mutations in NBEAL2, encodinga BEACH protein, cause gray platelet syndrome. Nat Genet. 2011;43(8):738-740.

4. Manchev VT, Hilpert M, Berrou E, et al. A new form of macro-thrombocytopenia induced by a germ-line mutation in the PRKACGgene. Blood. 2014;124(16):2554-2563.

5. Monteferrario D, Bolar NA, Marneth AE, et al. A dominant-negativeGFI1B mutation in the gray platelet syndrome. N Engl J Med. 2014;370(3):245-253.

6. Nesin V, Wiley G, Kousi M, et al. Activating mutations in STIM1 andORAI1 cause overlapping syndromes of tubular myopathy and con-genital miosis. Proc Natl Acad Sci USA. 2014;111(11):4197-4202.

7. Levin C, Koren A, Pretorius E, et al. Deleterious mutation in the FYBgene is associated with congenital autosomal recessive small-plateletthrombocytopenia. J Thromb Haemost. 2015;13(7):1285-1292.

8. Fletcher SJ, Johnson B, Lowe GC, et al; UK Genotyping and Pheno-typing of Platelets Study Group. SLFN14 mutations underlie throm-bocytopenia with excessive bleeding and platelet secretion defects. J ClinInvest. 2015;125(9):3600-3605.

9. Noetzli L, Lo RW, Lee-Sherick AB, et al. Germline mutations in ETV6are associated with thrombocytopenia, red cell macrocytosis and pre-disposition to lymphoblastic leukemia. Nat Genet. 2015;47(5):535-538.

10. Stritt S, Nurden P, Turro E, et al; BRIDGE-BPD Consortium. A gain-of-function variant in DIAPH1 causes dominant macrothrombocytopeniaand hearing loss. Blood. 2016;127(23):2903-2914.

11. Turro E, Greene D, Wijgaerts A, et al; BRIDGE-BPD Consortium.A dominant gain-of-function mutation in universal tyrosine kinase SRCcauses thrombocytopenia, myelofibrosis, bleeding, and bone pathologies.Sci Transl Med. 2016;8(328):328ra30.

12. Noris P, Biino G, Pecci A, et al. Platelet diameters in inherited throm-bocytopenias: analysis of 376 patients with all known disorders. Blood.2014;124(6):e4-e10.

13. Balduini CL, Melazzini F, Pecci A. Inherited thrombocytopenias-recentadvances in clinical and molecular aspects. Platelets. 2017;28(1):3-13.

14. Pecci A, Klersy C, Gresele P, et al. MYH9-related disease: a novelprognostic model to predict the clinical evolution of the disease based ongenotype-phenotype correlations. Hum Mutat. 2014;35(2):236-247.

15. Noris P, Perrotta S, Bottega R, et al. Clinical and laboratory features of103 patients from 42 Italian families with inherited thrombocytopeniaderived from the monoallelic Ala156Val mutation of GPIba (Bolzanomutation). Haematologica. 2012;97(1):82-88.

16. Noris P, Perrotta S, Seri M, et al. Mutations in ANKRD26 are responsiblefor a frequent form of inherited thrombocytopenia: analysis of 78 patientsfrom 21 families. Blood. 2011;117(24):6673-6680.

17. Pleines I, Woods J, Chappaz S, et al. Mutations in tropomyosin 4 underliea rare form of human macrothrombocytopenia. J Clin Invest. 2017;127(3):814-829.

18. Savoia A, Kunishima S, De Rocco D, et al. Spectrum of the mutations inBernard-Soulier syndrome. Hum Mutat. 2014;35(9):1033-1045.

19. Kunishima S, Okuno Y, Yoshida K, et al. ACTN1 mutations causecongenital macrothrombocytopenia. Am J Hum Genet. 2013;92(3):431-438.

20. Guerrero JA, Kyei M, Russell S, et al. Visualizing the von Willebrandfactor/glycoprotein Ib-IX axis with a platelet-type von Willebrand dis-ease mutation. Blood. 2009;114(27):5541-5546.

21. Nurden AT, Pillois X, Fiore M, Heilig R, Nurden P. Glanzmannthrombasthenia-like syndromes associated with macrothrombocytopeniasand mutations in the genes encoding the aIIbb3 integrin. Semin ThrombHemost. 2011;37(6):698-706.

22. Kunishima S, Kobayashi R, Itoh TJ, Hamaguchi M, Saito H. Mutationof the beta1-tubulin gene associated with congenital macrothrom-bocytopenia affecting microtubule assembly. Blood. 2009;113(2):458-461.

Figure 3. Approach to the management of bleeding episodes inhereditary thrombocytopenia. rFVIIa, recombinant activated factor VII.

Hematology 2017 397

mailto:[email protected]

23. Morison IM, Cramer Borde EM, Cheesman EJ, et al. A mutation ofhuman cytochrome c enhances the intrinsic apoptotic pathway but causesonly thrombocytopenia. Nat Genet. 2008;40(4):387-389.

24. Stevenson WS, Rabbolini DJ, Beutler L, et al. Paris-Trousseau throm-bocytopenia is phenocopied by the autosomal recessive inheritance of aDNA-binding domain mutation in FLI1. Blood. 2015;126(17):2027-2030.

25. Noris P, Marconi C, De Rocco D, et al. A new form of inheritedthrombocytopenia due to monoallelic loss of function mutation in thethrombopoietin gene [published online ahead of print 3 May 2017]. Br JHaematol.

26. Stritt S, Nurden P, Favier R, et al. Defects in TRPM7 channel functionderegulate thrombopoiesis through altered cellular Mg(21) homeostasisand cytoskeletal architecture. Nat Commun. 2016;7:11097.

27. Massaad MJ, Ramesh N, Geha RS. Wiskott-Aldrich syndrome: a com-prehensive review. Ann N Y Acad Sci. 2013;1285:26-43.

28. Albert MH, Bittner TC, Nonoyama S, et al. X-linked thrombocytopenia(XLT) due to WAS mutations: clinical characteristics, long-term out-come, and treatment options. Blood. 2010;115(16):3231-3238.

29. Favier R, Akshoomoff N, Mattson S, Grossfeld P. Jacobsen syndrome:advances in our knowledge of phenotype and genotype. Am J Med GenetC Semin Med Genet. 2015;169(3):239-250.

30. Nurden P, Debili N, Coupry I, et al. Thrombocytopenia resulting frommutations in filamin A can be expressed as an isolated syndrome. Blood.2011;118(22):5928-5937.

31. Millikan PD, Balamohan SM, Raskind WH, Kacena MA. Inheritedthrombocytopenia due to GATA-1 mutations. Semin Thromb Hemost.2011;37(6):682-689.

32. Toriello HV. Thrombocytopenia-absent radius syndrome. Semin ThrombHemost. 2011;37(6):707-712.

33. Markello T, Chen D, Kwan JY, et al. York platelet syndrome is a CRACchannelopathy due to gain-of-function mutations in STIM1. Mol GenetMetab. 2015;114(3):474-482.

34. Ballmaier M, Germeshausen M. Congenital amegakaryocytic throm-bocytopenia: clinical presentation, diagnosis, and treatment. SeminThromb Hemost. 2011;37(6):673-681.

35. Albers CA, Paul DS, Schulze H, et al. Compound inheritance of a low-frequency regulatory SNP and a rare null mutation in exon-junctioncomplex subunit RBM8A causes TAR syndrome. Nat Genet. 2012;44(4):435-439.

36. Niihori T, Ouchi-UchiyamaM, Sasahara Y, et al. Mutations in MECOM,encoding oncoprotein EVI1, cause radioulnar synostosis with amega-karyocytic thrombocytopenia. Am J Hum Genet. 2015;97(6):848-854.

37. Liew E, Owen C. Familial myelodysplastic syndromes: a review of theliterature. Haematologica. 2011;96(10):1536-1542.

38. Kaushansky K. Thrombopoiesis. Semin Hematol. 2015;52(1):4-11.39. Hirata S, Takayama N, Jono-Ohnishi R, et al. Congenital amegakar-

yocytic thrombocytopenia iPS cells exhibit defective MPL-mediatedsignaling. J Clin Invest. 2013;123(9):3802-3814.

40. Sakurai M, Kasahara H, Yoshida K, et al. Genetic basis of myeloidtransformation in familial platelet disorder/acute myeloid leukemia pa-tients with haploinsufficient RUNX1 allele. Blood Cancer J. 2016;6:e392.

41. Bluteau D, Balduini A, Balayn N, et al. Thrombocytopenia-associatedmutations in the ANKRD26 regulatory region induce MAPK hyper-activation. J Clin Invest. 2014;124(2):580-591.

42. Songdej N, Rao AK. Hematopoietic transcription factor mutations:important players in inherited platelet defects. Blood. 2017;129(21):2873-2881.

43. Pecci A, Balduini CL. Lessons in platelet production from inheritedthrombocytopenias. Br J Haematol. 2014;165(2):179-192.

44. Bury L, Falcinelli E, Chiasserini D, Springer TA, Italiano JE Jr, GreseleP. Cytoskeletal perturbation leads to platelet dysfunction and throm-bocytopenia in variant forms of Glanzmann thrombasthenia. Haema-tologica. 2016;101(1):46-56.

45. Ong L, Morison IM, Ledgerwood EC. Megakaryocytes from CYCSmutation-associated thrombocytopenia release platelets by both

proplatelet-dependent and -independent processes. Br J Haematol. 2017;176(2):268-279.

46. Bluteau D, Glembotsky AC, Raimbault A, et al. Dysmegakaryopoiesis ofFPD/AML pedigrees with constitutional RUNX1 mutations is linked tomyosin II deregulated expression. Blood. 2012;120(13):2708-2718.

47. MaoGF, Goldfinger LE, Fan DC, et al. Dysregulation of PLDN (pallidin)is a mechanism for platelet dense granule deficiency in RUNX1 hap-lodeficiency. J Thromb Haemost. 2017;15(4):792-801.

48. Thon JN, Macleod H, Begonja AJ, et al. Microtubule and cortical forcesdetermine platelet size during vascular platelet production. Nat Commun.2012;3:852.

49. Verver EJ, Topsakal V, Kunst HP, et al. Nonmuscle myosin heavy chainIIAmutation predicts severity and progression of sensorineural hearing lossin patients with MYH9-related disease. Ear Hear. 2016;37(1):112-120.

50. Melazzini F, Palombo F, Balduini A, et al. Clinical and pathogenicfeatures of ETV6-related thrombocytopenia with predisposition to acutelymphoblastic leukemia. Haematologica. 2016;101(11):1333-1342.

51. Arber DA, Orazi A, Hasserjian R, et al. The 2016 revision to the WorldHealth Organization classification of myeloid neoplasms and acuteleukemia. Blood. 2016;127(20):2391-2405.

52. Noris P, Schlegel N, Klersy C, et al; European Hematology Association –Scientific Working Group on Thrombocytopenias and Platelet FunctionDisorders. Analysis of 339 pregnancies in 181 women with 13 differ-ent forms of inherited thrombocytopenia. Haematologica. 2014;99(8):1387-1394.

53. Balduini CL, Savoia A, Seri M. Inherited thrombocytopenias frequentlydiagnosed in adults. J Thromb Haemost. 2013;11(6):1006-1019.

54. Pecci A. Diagnosis and treatment of inherited thrombocytopenias. ClinGenet. 2016;89(2):141-153.

55. Johnson B, Lowe GC, Futterer J, et al; UK GAPP Study Group. Wholeexome sequencing identifies genetic variants in inherited thrombocy-topenia with secondary qualitative function defects. Haematologica.2016;101(10):1170-1179.

56. Pecci A, Granata A, Fiore CE, Balduini CL. Renin-angiotensin systemblockade is effective in reducing proteinuria of patients with progressivenephropathy caused by MYH9 mutations (Fechtner-Epstein syndrome).Nephrol Dial Transplant. 2008;23(8):2690-2692.

57. De Rocco D, Melazzini F, Marconi C, et al. Mutations of RUNX1 infamilies with inherited thrombocytopenia. Am J Hematol. 2017;92(6):E86-E88.

58. Bolton-Maggs PH, Chalmers EA, Collins PW, et al; UKHCDO. A reviewof inherited platelet disorders with guidelines for their management onbehalf of the UKHCDO. Br J Haematol. 2006;135(5):603-633.

59. Estcourt LJ, Birchall J, Allard S, et al; British Committee for Standardsin Haematology. Guidelines for the use of platelet transfusions. Br JHaematol. 2017;176(3):365-394.

60. Kaufman RM, Djulbegovic B, Gernsheimer T, et al; AABB. Platelettransfusion: a clinical practice guideline from the AABB. Ann InternMed. 2015;162(3):205-213.

61. Orsini S, Noris P, Bury L, et al; European Hematology Association -Scientific Working Group (EHA-SWG) on thrombocytopenias andplatelet function disorders. Bleeding risk of surgery and its prevention inpatients with inherited platelet disorders. Haematologica. 2017;102(7):1192-1203.

62. Pecci A, Gresele P, Klersy C, et al. Eltrombopag for the treatment of theinherited thrombocytopenia deriving from MYH9 mutations. Blood.2010;116(26):5832-5837.

63. Pecci A, Barozzi S, d’Amico S, Balduini CL. Short-term eltrombopag forsurgical preparation of a patient with inherited thrombocytopenia de-riving fromMYH9mutation. ThrombHaemost. 2012;107(6):1188-1189.

64. Favier R, Feriel J, Favier M, Denoyelle F, Martignetti JA. First successfuluse of eltrombopag before surgery in a child with MYH9-relatedthrombocytopenia. Pediatrics. 2013;132(3):e793-e795.

65. Fiore M, Saut N, Alessi MC, Viallard JF. Successful use of eltrombopagfor surgical preparation in a patient with ANKRD26-related thrombo-cytopenia. Platelets. 2016;27(8):828-829.


66. Gerrits AJ, Leven EA, Frelinger AL III, et al. Effects of eltrombopag onplatelet count and platelet activation in Wiskott-Aldrich syndrome/X-linked thrombocytopenia. Blood. 2015;126(11):1367-1378.

67. Hacein-Bey Abina S, Gaspar HB, Blondeau J, et al. Outcomes followinggene therapy in patients with severe Wiskott-Aldrich syndrome. JAMA.2015;313(15):1550-1563.

68. Oshima K, Imai K, Albert MH, et al. Hematopoietic stem cell trans-plantation for X-Linked thrombocytopenia with mutations in the WASgene. J Clin Immunol. 2015;35(1):15-21.

69. Rieger C, Rank A, Fiegl M, et al. Allogeneic stem cell transplantation asa new treatment option for patients with severe Bernard-Soulier Syn-drome. Thromb Haemost. 2006;95(1):190-191.

70. Moratto D, Giliani S, Bonfim C, et al. Long-term outcome and lineage-specific chimerism in 194 patients with Wiskott-Aldrich syndrometreated by hematopoietic cell transplantation in the period 1980-2009: aninternational collaborative study. Blood. 2011;118(6):1675-1684.

71. Shin CR, Kim MO, Li D, et al. Outcomes following hematopoietic celltransplantation forWiskott-Aldrich syndrome. BoneMarrow Transplant.2012;47(11):1428-1435.

72. Dalle JH, Fahd M. Allogeneic stem cell transplantation in amegacar-yocytosis: results of a retrospective study in EBMT centers. Biol BloodMarrow Transplant. 2014;20(2)(suppl):S81-S82.

73. Braun CJ, Boztug K, Paruzynski A, et al. Gene therapy for Wiskott-Aldrich syndrome–long-term efficacy and genotoxicity. Sci Transl Med.2014;6(227):227ra33.

74. Aiuti A, Biasco L, Scaramuzza S, et al. Lentiviral hematopoietic stem cellgene therapy in patients with Wiskott-Aldrich syndrome. Science. 2013;341(6148):1233151.

75. Pala F, Morbach H, Castiello MC, et al. Lentiviral-mediated gene therapyrestores B cell tolerance in Wiskott-Aldrich syndrome patients. J ClinInvest. 2015;125(10):3941-3951.

Hematology 2017 399

hereditary thrombocytopenias: a growing list of...

Documents